This invention relates generally to thermal transfer systems, and more particularly to a sealed pipe containing a working fluid which is alternately evaporated and condensed to transfer heat.
A heat pipe comprises a sealed envelope containing a working fluid having both a liquid phase and a vapor phase which is the desired range of operating temperatures. When one portion of the envelope is exposed to a relatively higher temperature, it functions as an evaporator section. The working fluid is vaporized in the evaporator section and flows in the vapor phase to the relatively lower temperature section of the envelope which becomes a condenser section. The working fluid is condensed in the condenser section and then returns in the liquid phase in a short time from the higher temperature section of the envelope to the lower temperature section as a consequence of the phase change of the working fluid.
Because it operates on the principle of phase change rather than on the principles of conduction or convection, a heat pipe is theoretically capable of transferring heat at a much higher rate than conventional heat transfer systems. Nevertheless, a number of difficulties have heretofore been experienced in attempting to use heat pipes in commercially attractive applications. For example, until quite recently the only heat pipes actually in operation utilized a capillary wick to transport the liquid phase longitudinally of the pipe from the evaporator section to the condenser section. In heat pipes using a wick the quantity of working fluid is selected so that no surplus liquid phase is provided at the desired operating temperature. As a result there is only modest interference between the liquid phase and the vapor phase. However, capillary wicks are difficult and expensive to install properly, and for this reason the use of heat pipes incorporating such wicks has been limited to special and very expensive applications such as in nuclear reactors and space craft.
If a heat pipe envelope is generally tubular in shape and is disposed substantially horizontally, the liquid phase of the working fluid will return to the high temperature of the heat pipe in either direction under the action of gravity so that heat transfer is bidirectional and does not require a capillary wick to return the working fluid to the evaporative section, thus permitting a more inexpensive heat pipe to be used. However, this type of heat pipe exhibits a particular problem which heretofore has limited its heat transfer capability to rates considerably below the theoretical level in the absence of such liquid entrainment. The problem is that even at relatively low heat transfer rates the liquid phase of the working fluid returns from the condenser portion to the evaporator portion of the heat pipe in a series of waves. These waves tend to interfere with the flow of the vapor phase of the working fluid from the evaporator portion to the condenser portion of the heat pipe, and thereby tend to limit the heat transfer capability of the heat pipe. At high heat transfer rates, the waves reach sufficient magnitude to form slugs which completely block flow of the vapor phase back to the condenser, thus substantially killing transfer of heat by phase change since as one slug disappears, another slug forms.
A closed cycle heat transfer system similar to a heat pipe was first disclosed in British Pat. No. 22,272 granted to Perkins et al on Dec. 5, 1892. However, efforts to use the so-called Perkins tube has been limited to application in which very short tubes are used, probably because the systems proposed by Perkins are not sufficiently effective to be of interest commercially, as will hereinafter be shown in greater detail.